Refining a contract -- state responsibilities and obligations

• invariance -- respect the invariants of the base class
• methods -- services may be added or refined

Refining a method -- like improving a business contract

  class C : public P {
  	virtual void m();
  	}
  

• pre($m\_C$) $>=$ pre($m\_P$)

  weaken pre-condition

  
  
• post($m\_C$) post($m\_P$)

  strengthen post-condition